The performance of micro-electronic devices has always been limited by the variations found in the dimensions of their critical features, termed critical dimensions or CD. Micro-electronic devices are often manufactured using masks (or reticles) in a photolithography process. The latter is one of the principal processes in the manufacture of semiconductor devices, and consists of patterning the wafer's surface in accordance with the circuit design of the semiconductor devices to be produced. Such a circuit design is first patterned on a mask. Hence, in order to obtain operating semiconductor devices, the mask must be defect free. Moreover, the mask is often used in a repeated manner to create many dies on the wafer. Thus, any defect on the mask will be repeated multiple times on the wafer and will cause multiple devices to be defective. Establishing a production-worthy process requires tight control of the overall lithography process. Within this process, CD control is a determining factor with respect to device performance and yield.
When the critical dimensions are large, systematic variations in the dimensions of the device, such as those caused by material physics or as a result of equipment or the production process, do not make large contributions to the overall error budget and can therefore be largely ignored. However, as the minimum size of critical features drops below about 65 nm, systematic variations that were previously ignored can now consume a considerable portion of the overall error budget. Specifically, systematic mask CD errors can consume over 50% of the total wafer lithography process CD budget.
Therefore, various mask inspection tools have been developed and are available commercially. According to the known techniques of designing and evaluating masks, the mask is created and used to expose therethrough a wafer, and then a check is performed to determine whether the features of the mask have been transferred to the wafer according to the design. Any variations in the final features from the intended design necessitate modifying the design, creating a new mask, and exposing a new wafer.
The above procedure can be made simpler using the Aerial image Measurement System (AIMS). The AIMS is basically an engineering tool, which is intended for the development and testing of various mask designs. It is also helpful for checking how Optical Proximity Correction (OPC) and phase shift features would print on the wafer. Additionally, the system can be used to study various defects discovered by a mask inspection system, and test whether those defects would actually print on the wafer. Some systems have been developed using the principles of aerial imaging for the mask inspection, as disclosed for example in U.S. Pat. Nos. 5,481,624; 5,795,688; and 7,072,502. Also, the use of aerial imaging in the mask inspection is described in the article “Aerial-image-based off-focus inspection: lithography process window analysis during mask inspection”, Shirley Hemar et al., Proceedings of SPIE, Volume 5256, 23rd Annual BACUS Symposium on Photomask Technology, December 2003, pp. 500-509.
Generally speaking, the AIMS™ is an optical system for evaluating masks under specific stepper or scanner settings of numerical aperture (NA), partial coherence of illumination or pupil filling, wavelength and illumination type (like circular, annular, quadrupole or dipole off-axis illumination). By flexible, automated adjustment of any setting to match conditions like in 193 nm exposure tools, it can emulate for any type of masks like binary, OPC and phase shift, designed for 193 nm lithography. The image taken with the system is optically equivalent to the latent image incident on the photoresist of the wafer, but magnified and recorded with a Charged Couple Device (CCD) camera. Thus, the AIMS™ tool allows a rapid prediction of the wafer printability of critical features, like dense patterns or contacts, defects or repairs on the mask without the need to do real wafer prints using the exposure tool and a following Scanning Electron Microscope (SEM) measurement of the printed features.
There is a need to provide systems and methods for evaluating at least one parameter of a pattern.